Pigments which are produced by way of physical vapor deposition (PVD) of single-layer or multi-layer films on a carrier, subsequent detachment and then crushing of the films or film packs are becoming of increasing interest in very recent times because of their special optical properties. Thus for example single-layer aluminum pigment which is produced by way of PVD and which is known under the name METALURE (registered mark) is highly valued in the printing and paints industry because of its outstanding mirror shine while multi-layer so-called optically variable pigments which have brilliant interference colors and which produce pronounced angle-dependent color shade variations are increasingly used in paints, plastic materials and in bond printing.
The single-layer aluminum scales or flakes with a very high degree of shine of the above-mentioned aluminum pigment are produced by vacuum deposition on a substrate provided with a release layer, subsequent detachment of the aluminum film, and mechanical crushing thereof. The thickness of the film particles is generally less than 100 nm. The scale or flake surface is mirror-smooth and is of the highest level of perfection. The flake or scale surface can however also have a hologram-like embossing (WO 93/23481). Depending on the embossing involved such flakes appear in variable colors.
The basic structure of optically variable multi-layer pigments is as follows: following a central highly reflective metal layer M, towards each side, is a transparent low-refractive layer T and then a semi-transparent metal layer M'. Films which involve a multi-layer structure of the type M'TM have been known for many years (see Optical Acta 20, 925-937 (1973) and U.S. Pat. No. 3,858,977). Pigments which embody the optical principle which is applicable in respect of films and which have the above-mentioned layer sequence M'TMTM' were first described in U.S. Pat. No. 3,438,796. The flake-like pigment particles, envisaged for the area of use of decorative paints and lacquers, exhibit brilliant colors and comprise a highly reflective, central aluminum layer which is at least 60 nm in thickness and which is accompanied in an outward direction by a respective SiO.sub.2 -layer which is between 100 and 600 nm in thickness and which is then followed by a semi-transparent aluminum layer which is between 5 and 40 nm in thickness. Thereover there is also an SiO.sub.2 -protective layer. The production of such pigments is effected by vapor deposition of subsequent layers and then crushing of the multi-layer film to the particle size of special-effect pigments. So as to facilitate detachment of the film from the substrate it is covered with a release layer. The color of the pigments produced in that way depends on the thickness of the SiO.sub.2 -layers. Each color shade of the spectrum can be specifically set by way of the choice of the thickness of the SiO.sub.2 -layers. Higher-order interference colors are also possible.
Pigments involving a similar structure and a similar production process are described in U.S. Pat. No. 5,135,812 and EP-A 227 423. Those pigments have a multi-layer structure, wherein the central opaque layer comprises a highly reflective metal layer, generally aluminum, the transparent layers which follow it in an outward direction comprise MgF.sub.2 or SiO.sub.2 (refractive index n&lt;1.65) and a semi-transparent or semi-opaque metal layer. The area of use is printing inks for forgery-resistant banknotes. The pigments are produced by physical vapor deposition in a vacuum and then crushing of the multi-layer film, generally in an ultrasonic crusher, to pigment particle size.
The pigments described in the above-indicated patents suffer from the disadvantage that they are susceptible to corrosion by virtue of exposed metal surfaces. Admittedly the pigments which are produced from films manufactured by vapor deposition in a multi-layer structure can already be passivated on the large surfaces of the pigment particles by the production of a protective layer in the vapor deposition process, as was effected in accordance with U.S. Pat. No. 3,438,796. The operation of crushing the multi-layer film however also gives rise to fresh fracture locations which, due to the procedure involved, are unprotected and are therefore highly sensitive to corrosion. Particularly in the presence of moisture, acids or bases, the chemical reactivity of the fresh fracture locations results in corrosion and thus inevitably results in an impairment in the brilliance and coloristics of the pigment scales or flakes. That represents a serious problem in terms of technical application. U.S. Pat. No. 5,498,781 described an initial attempt at passivating optically variable pigments for aqueous coating systems. The ready-for-sale, optically variable pigment powder was thereafter surface-coated with a silane compound of the type R.sub.3 Si--A--X, specifically (CH.sub.3 CH.sub.2 O).sub.3 Si(CH.sub.2).sub.3 NH.sub.2 in an aqueous alcohol solution, tempered at 110.degree. C. and then reacted in an alcohol solution with a polymer bearing functional isocyanate groups.
The result is a finished lacquer or paint as is used for the decorative surface coating of motor vehicles. Thus the passivation operation described in U.S. Pat. No. 5,498,781 leads to a single system of use and lacks the multiple and varied use options of a passivated pigment. A further disadvantage of that passivation process is that the corrosion processes have already begun at the endangered fracture edges of the optically variable pigments when treatment of the finished pigment powder begins.